fix(a380/mfd): FLT NBR is now displayed as a mandatory input field

.github/CHANGELOG.md

@@ -4,6 +4,7 @@
 <!-- Always use "1." at the start instead of "2. " or "X. " as GitHub will auto renumber everything. -->
 <!-- Use the following format below -->
 <!--  1. [Changed Area] Title of changes - @github username (Name)  -->
+1. [A380X/MFD] Fixed FLT NBR not displaying as a mandatory input field - @jet2code (john.xyz)
 
 ## 0.13.0
 

fbw-a380x/src/systems/instruments/src/MFD/FMC/fmgc.ts

@@ -57,7 +57,7 @@ export class FmgcData {
 
   public readonly cpnyFplnUplinkInProgress = Subject.create(false);
 
-  public readonly atcCallsign = Subject.create<string>('----------');
+  public readonly atcCallsign = Subject.create<string | null>(null);
 
   /** in degrees celsius. null if not set. */
   public readonly cruiseTemperaturePilotEntry = Subject.create<number | null>(null);